In Electronics world? Bipolar junction Transistors. Easily.
This led into having portable devices we have today.
Back then people used vacuum tubes for switching and amplification; of which were very expensive to run (used a lot of power when idle, while having a very short lifespan of less than 48 hrs).
I mean, vacuum tubes where phenomenal when they came, allowed first long distance calls in 1915.
Look at my phone now, fits on my hands, and has billions of transistors!
Post script: lately I've been thinking, what if we remove cell towers as middle men? Because nowadays privacy is somewhat dead. People have been using radio frequency for walkie-talkies even before 1st generation communication (1G) was a thing.
I was thinking the photolithography process might be almost as important as the transistor itself. Without the ability to miniaturize transistors and create integrated circuits, we wouldn't have anywhere near the level of technology we can build now. A computer made of discrete transistors would be way more efficient, reliable, and cheaper than one made with vacuum tubes, but would still be very limited. There are things you fundamentally couldn't do with even thousands of discrete transistors that became possible once we were able to scale to millions and now billions.